This invention relates to an antiskid brake control apparatus for controlling the skidding of the wheels of a vehicle at the time of braking.
Antiskid brake control involves repeating the operations of detecting skidding of a wheel at braking through use of an electronic control unit, eliminating skidding by reducing brake pressure and then subsequently increasing braking pressure again, thereby stabilizing steering of the vehicle at braking and making the braking distance as short as possible.
An example of a control apparatus for performing antiskid brake control of this kind is disclosed in the specification of U.S. Pat. No. 4,715,666. The disclosed antiskid brake control apparatus includes a normally-closed two-way solenoid valve which decreases brake pressure by opening and increases brake pressure by closing, and a metering spool valve which, at a decrease in pressure, shuts a passage directly communicating a wheel cylinder and a master cylinder and communicates the passage to the intake side of a pump via the open two-way solenoid valve, and which reduces the slope of a pressure increase when pressure is increased again.
The metering spool valve has a fixed orifice formed in a metering spool, and a variable orifice including a port and a land which is formed on the metering spool. At a decrease in pressure, brake fluid from the master cylinder and brake fluid discharged from the pump flow through the fixed orifice and variable orifice to produce a differential pressure by which the metering spool is moved. The metering spool is positioned at a location at which the pressure differential and the biasing force of a spring balance each other.
In the antiskid brake control apparatus described above, the pump is driven when antiskid braking control is applied. However, when the pressure is decreased at opening of the two-way solenoid valve, the discharge side and intake side of the pump are communicated via the fixed orifice and variable orifice, and therefore the load upon the pump takes on a large value. As a consequence, it is necessary to increase the discharge capacity of the pump. The result is a pump of large size.
For this reason, an antiskid brake control apparatus has been developed in which a three-way solenoid changeover valve is used to completely cut off the discharge and intake sides of the pump at a decrease in pressure. In accordance with this antiskid brake control apparatus, a large burden on the pump is avoided when pressure is decreased. However, since this antiskid brake control apparatus employs the three-way solenoid changeover valve, there are two valve-seat locations for the ball constituting the valve body, and the valve passages are complicated as well. In addition, the overall control apparatus is large in size and complicated in structure.